This invention relates to a structure for mounting a semiconductor device to a circuit board by using a tape carrier film and also to a structure for mounting a flip chip having bump electrodes on its surface to a circuit board to electrically connect them through a tape carrier film.
When a plurality of integrated circuits are combined and driven as a system, the individual IC chips are separately packaged and these packages are then electrically connected to a circuit board. This conventional method of mounting is not effective for reducing the size of electronic devices because the density of mounted devices cannot be increased beyond a certain limit if the devices are already packaged individually. If the density of mounted devices cannot be increased, furthermore, the lengths of wires connecting them cannot be reduced beyond a certain limit and this affects the performance characteristics of the system adversely.
In view of the above, the so-called multi-chip package, wherein a plurality of devices are mounted within a single package, has come to be considered in recent years as an effective method of overcoming these difficulties. Some of the already considered methods of mounting a plurality of devices in a multi-chip package are the wire bonding method, the flip chip method and the film carrier method. Of the above, the most effective seems to be the film carrier method which is a type of the so-called gang bonding method whereby a plurality of electrodes are bonded at once. This method is also preferable because it is suited for mass production of these devices.
To describe the film carrier method by way of FIG. 7-A, outer leads 2 which are designed to correspond to the pads of an IC are preliminarily formed on a tape carrier film 1 such that they extend into an opening 3 of a generally square shape. A semiconductor device 4 cut out from a wafer is placed in this opening 3 and each of these outer leads 2 is electrically connected to the device 4 by inner lead bonding. Thereafter, the outer leads 2 are cut off as shown in FIG. 7-B and the device 4 is removed from the tape carrier film 1. Each outer lead 2 may be bent as shown in FIG. 7-C, if necessary, and then electrically connected as shown in FIG. 7-D to a wiring pattern 6 formed on a circuit board 5.
When a plurality of semiconductor devices are connected by the aforementioned film carrier method for mounting to a circuit board in a multi-chip package, the devices are distributed two-dimensionally, or in a plane, as shown in FIG. 8 with the ends of their outer leads 2 connected to terminal electrodes 7 of the wiring pattern 6 on the circuit board 5. Accordingly, there must be reserved on the circuit board 5 an area which corresponds to the number of the semiconductor devices 4 to be mounted thereon. Although the circuit board 5 can be made smaller by this method than if individually packaged devices were mounted, the overall size of the package cannot be reduced by this method beyond a certain limit. Moreover, since the wiring pattern 6 for connecting the devices 4 must be formed according to the positions of the devices 4 within the package, failure to reduce the size of the circuit board means adverse effects on the device characteristics such as delay in transmission and cross talk especially in the case of semiconductor devices of a high-speed response type.
In view of the above, flip chips are considered favorable because not only can a flip chip be mounted by means of bumps formed on active areas but electrodes (bumps) can be pulled out freely from any desired positions. Thus, the flip chip enjoys a large degree of freedom in its design and since bumps can be formed all over its surface, it is particularly suited for multi-terminal connection. Moreover, since the flip chip, unlike the wire bonding method, does not require a special area for connection, chips can be made smaller and the production cost can be reduced and this makes the flip chip method even more attractive for the mounting of high-speed devices. According to a general method illustrated in FIG. 9 of attaching a flip chip, however, bumps 22 which are formed by soldering on the surface of a chip 21 are directly soldered onto terminal electrodes 24, 24' etc. of the circuit board 23 of a package. For this reason, the chip 21 becomes heated and the general temperature changes in the environment cause distortion between the chip 21 and the circuit board 23 due to their difference in thermal expansion. This is particularly serious near the periphery of the chip 21 because connections between the bumps 22 and terminal electrodes 24' in this region are very easily breakable.
Use of ceramic materials such as silicon carbide and aluminum nitride with coefficients of thermal expansion nearly equal to that of the chip 21, or silicon, has been considered for the circuit board such that the adverse effects of the difference in thermal expansion can be made negligible, but since this method is expensive and involves electrical problems, it has not been considered practicable. Another way to obviate the problems described above is to use the so-called area tape automated bonding (area TAB) method according to which the flip chip 21 is not directly mounted on the circuit board 23 but is bonded first to a tape carrier film of a flexible film-like material and then mounted to the circuit board 23 through this film. To explain this method in detail by way of FIG. 10, pads 26 are preliminarily prepared on a tape carrier film 25 corresponding to the positions of bumps 22 on the flip chip 21 and each bump 22 of the flip chip 21 which is cut off from a wafer is connected by bonding to a pad 26 on the tape carrier film 25. Thereafter, leads 28 extending radially from the pads 6 to make connections across a square-shaped opening 27 are cut along its edges as shown in FIG. 10-B and the flip chip 21 to which the tape carrier film 25 is attached is connected as shown in FIG. 10-C by connecting each lead 28 to a terminal electrode 24 on the circuit board 23. If this mounting method is used, the flexible tape carrier film 25, which is sandwiched between the flip chip 21 and the circuit board 23, can absorb the distortion caused by the difference in thermal expandion between the chip 21 and the circuit board 23 such that a highly reliable product can be obtained. The tape carrier method is also adapted for mass production.
Although the tape carrier method is generally effective as described above, each electric signal from the flip chip 21 must be transmitted first through the wiring on the tape carrier film 25 to a lead 28 around a pad 26 on the film 25 and then to a desired circuit on the circuit board 23 through this lead 28. If there are very many bumps 22 provided all over the surface of the flip chip 21 as is often the case as shown, for example, in FIG. 10-A, it becomes impossible to take out all signals from the chip. Even if leads 28' are additionally provided as shown in FIG. 11 on the opposite side of the tape carrier film 25 and are each electrically connected to a pad 26 through a throughhole 29, the number of internally located pads from which signals can be extracted is limited, as shown in FIG. 12, by the number of lead lines 28 which can pass between two mutually adjacent pads 26 adjacent to the periphery. When there are very many pads and hence many pads at inner locations, therefore, signals cannot be taken out from some of the inner pads 26'.